FEM solvers break up a large design into lots of tiny meshes, then solve integrals for the electrical/magnetic fields. At high frequency, this is significantly more accurate than approximating based on the copper geometry.

Two big uses are antenna design and RF circuit design.

I can't comment on this specific solver but it does mention using EMerge, an existing FOSS project, as its core. Another FOSS alternative is openEMS. Anyone doing this for a job will just pay for CST/Sonnet/HFSS/Axiem/EMPro etc.